This application relates to the art of valves and, more particularly, to valve stem packing assemblies for quarter turn valves such as ball, plug or butterfly valves.
The two purposes of stem packing assemblies for quarter turn, half turn and similar valves are to seal the passage through which the rotatable stem penetrates the valve body and to provide a bearing for carrying the internal pressure load acting on the stem. These two purposes apply to stem packing assemblies for all kinds of quarter turn, half turn and similar valves, including ball, plug or butterfly. The requirements differ from other valves in that the motion is pure rotation for only a partial turn and there is no axial motion.
The most basic stem packing approach comprises an O-ring seal. Such a seal is simple to produce, small, compact, inexpensive, and effective in many non-demanding applications.
A more sophisticated approach uses a bushing of Teflon (a trademark of E. I. duPont de Nemours and Company for polytetrafluoroethylene materials). This is probably the most common type of construction, and means is typically provided for compressing the bushing. This approach requires a somewhat more bulky assembly, and is more expensive and more complex as compared to an O-ring. However, the Teflon bushing arrangement can be used with a wider variety of chemicals and usually over a much wider temperature range as compared to O-rings.
A third approach uses a high temperature material other than Teflon, such as asbestos or Grafoil (a trademark of Union Carbide Corporation for an expanded flexible carbonaceous material having no resin or organic binders). These arrangements are generally too expensive and not sufficiently reliable for general use, and are usually reserved for high temperature applications.
All of the above seal arrangements operate to seal the valve stem passage by imparting an axial force to the packing assembly to axially compress the packing materials to thereby force radially inward and outward deformation of the packing materials. Such deformation is designed to result in a sealing engagement of the packing with the stem and stem passage. The efficiency of the translation of the axial force into sealing radial expansion of the packing largely determines the effectiveness of the seal arrangement and the life of the seal over a number of valve cycling operations. Seal arrangements which particularly absorb axial force without radial deformation are undesirably inefficient and frequently demand adjustment to maintain the proper seal.
All of the above seal arrangements have certain limitations and undesirable characteristics depending upon the application. With O-rings, pressures and temperatures are restricted to moderate levels and useful ranges, and suitable materials for O-rings are incompatible with many chemicals and solvents.
Although Teflon is compatible with most chemicals, it is temperature limited. Asbestos or Grafoil can withstand high temperatures but are not as leaktight and reliable under normal conditions. Teflon, asbestos, and Grafoil wear out with cycling of the valve so the packing becomes loose and leaks, thus requiring frequent adjustment. Thermal cycling also causes the packings to become loose because the packing materials expand at different rates than metals. Teflon is particularly troublesome because it expands approximately ten times as fast as metal.
All of the above limitations exist with normal packing systems in normal service. When fire safety requirements are added, the limitations are much more serious because the packing must seal during and after a fire, and the valve must be operable, at least once or a few times, without leaking. The usual materials used for O-rings, Teflon and other nonmetallics, are destroyed during a fire, and are either completely consumed or leave only a charred residue. Even fire resistant materials can fail if the heat causes a small amount of shrinkage which, in turn, causes the packing to become loose.
Supporting the thrust load on the stem is done internally or externally, with internal support perhaps being the most common. In internal thrust loading, the stem includes a head having a shoulder for supporting a bearing. The stem is inserted from the inside of the body, and a bearing material is located between the stem shoulder and a flange on the body. This bearing carries the thrust load and allows the stem to rotate freely. The bearing arrangement is small, compact, inexpensive and the stem head makes the stem inherently blowout proof. The stem cannot blow out of the valve body under pressure even if all the external packing components are removed. However, internal bearings are subject to damage by process fluids.
A less common construction is an external thrust bearing, usually combined in some way with the stem packing. This requires some kind of external yoke construction which is larger, more expensive and, if it is removed or damaged in service, the stem can blow out under pressure. The main advantage is that it places the bearing outside the valve body, away from the system fluid, where it can be lubricated and constructed for maximum wear life. However, external bearings are exposed to contamination, dirt and corrosive atmospheres. In most quarter turn valves, the wear on the thrust bearing is rarely the factor which limits valve performance.
Bearing loads are also affected by packing adjustment. The packing load is usually applied by clamping against the bearing, and avoiding such clamping requires additional complex yoke devices on both internal and external bearing systems. Overtightening the packing, either by excessive manual adjustment or by thermal expansion, can overload the bearing. This situation causes even faster packing and bearing wear, shorter life, and further loosening.
It has been considered desirable to eliminate or minimize the foregoing limitations and undesirable characteristics in a simple, reliable, and inexpensive manner. The subject invention is considered to meet these needs and others by providing a new and improved valve stem packing assembly.